The present invention relates to a time-division multiplex communication system, and more particularly a new and improved communication system in which recurring time slots are each uniquely identified by simultaneously occuring synchronization signals and information is carried by a path consisting of a bundle of discrete time division lines.
Electronic communication systems were at one time based exclusively on space-division techniques. That is, electromagnetic switches were used to establish connections along discrete paths each path being dedicated solely to a particular connection throughout its duration. This required large numbers of interconnecting lines in relation to the total number of system inputs and outputs. Moreover, bulky, slow and expensive servo mechanisms were relied on to establish the desired connections.
With the advent of more sophisticated solid state switching devices and the availability of integrated circuit modules has come the use of time division for information transmission. This sampling technique enables a large number of users to communicate over a single line or bus while only selected interconnections among these users are established. The composite signal passing over the time-division line is divided into a series of frames. Each frame is divided into a series of time slots and each time slot occurs once per frame.
All communicating locations must be synchronized to a common time base so that the desired recurring time slots can be reliably identified. Synchronization is most commonly accomplished by a periodically occuring sync signal superimposed on the information carried by the time-division path, the sync pulses being identifiable by a predetermined characteristic such as amplitude superiority. Conventional synchronization techniques often require an oscillator or counter at each user location with periodic correction in response to the sync signals so that errors are not cumulative. A part of the signal-carrying capacity of the system must therefore be dedicated to the sync pulses, thus diminishing useful information capacity.
Most presently known time division communication systems are relatively complex and expensive, thus not permitting their usage for many purposes. They are generally predicted on the assumption that only a small percentage of possible user locations will be active at any one time, and the synchronization techniques conventionally employed require relatively elaborate circuitry at each location. When the percentage of possible simultaneous users approaches the total number of users connected to the system, the synchronization information becomes sufficiently complex and voluminous to greatly diminish the capacity of the system to transmit information. Efforts to simplify synchronization have, in the past, succeeded only at the expense of reliability.
Another disadvantage of presently known time division systems is that they are applicable only to situations in which an approximation of the transmitted signal based on a limited number of separated samples is adequate for the purpose of the system.